Method of producing an ultra-clean,bright surface on titanium

ABSTRACT

THIS INVENTION RELATES TO THE TREATMENT OF TITANIUM OR TITANIUM-BASE ALLOY ARTICLES FOR THE PURPOSE OF EFFECTING AN ULTRA-CLEAN, BRIGHT SURFACE THEREON. THE METHOD INVOLVES TREATING THE SURFACE WITH A SOLUTION OF MTHYL ALCOHOL, SLFURIC ACID, AND HYDROFLUORIC ACID WHICH IS MAINTAINED AT ABOUT -45* C. AND THROUGH WHICH IS PASSED AN ELECTRIC CURRENT.

United States Patent 3,795,597 METHOD OF PRODUCING AN ULTRA-CLEAN,

BRIGHT SURFACE 0N TITANIUM Owen M. Katz, Pittsburgh, and Charles E. Campbell,

Elizabeth, Pa., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Mar. 15, 1973, Ser. No. 341,617

- Int. Cl. C23b 3/02 US. Cl. 204-1415 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the treatment of titanium ortitanium-base alloy articles for the purpose of effecting an ultra-clean, bright surface thereon. The method involves treating the surface with a solution of methyl alcohol, sulfuric acid, and hydrofluoric acid which is maintained at about -45 C. and through which is passed an electric current.

CONTRACTU ORIGIN This invention evolved under Contract AT- ll-l -GEN- 14 between the United States Atomic Energy Commission and the Westinghouse Electric Corporation.

BACKGROUND OF INVENTION.

Highly polished, clean, titanium surfaces are essential in some applications of the metal, for example, toprepare thinned foils or sheets for transmission electron microscopy. In particular, it is a critical requirement for the titanium tubes that are used in heat exchangers. Prior art methods for electropolishing titanium and titanium alloys are revealed in US. Pats. 2,780,594, 2,997,429, and 3,030,286. These patents disclose the prior art electrolytic solutions and conditions for cleaning and polishing titanium and titanium alloys. However, the methods in these patents and other known methods never successfully produced a surface as clean and polished as obtained by practicing the subject invention.

SUMMARY OF THE INVENTION According to the present invention, we provide a method for treating titanium-base alloy articles to'efiect' an ultraclean, bright surface thereon. The method involves making said article the anode of an electrolyte solution which is made up of about 97% methyl alcohol, 2% sulfuric acid, and 1% hydrofluoric acid by volume, and which is kept at about -45 C. and applying a voltage across the solution. The titanium article is exposed to these conditions which also includes agitation for a time period that is contingent upon the amount of current passing through the solution. Thus, this invention contemplates a low temperature, electrolytic method, employing a moving bath of unique composition to achieve a highly reflective, ultra-clean surface on titanium or titanium alloys.

DETAILED DESCRIPTION OF THE INVENTION There are a number of variables in the method which can be changed and still effect a bright finish on the titanium tube, sheet, or foil material. These variables are electrolyte composition, temperature, voltage, current density, and polishing time.

In conjunction with other variables, electrolyte compositions of the acids can be varied. It was found that sulfuric acid concentration can be varied from a maximum of 4% to a minimum of 1 /2%, and hydrofluoric acid concentration can be varied from a maximum of 2% to a minimum of 3,795,597 Patented Mar. 5, 1974 See C. and -20? C. acceptable results depend upon the stirring rate. A fairly fast stirring rate will somewhat compensate for the disadvantage inherent with high temperatures. Thus, the best product will be obtained if the solution is agitatedduring the course of the electrolysis.

Voltage, current density (computed from the current flowing divided by the area of sample being polished), and polishing time are interrelated and should all be specified to obtain a bright, ultra-clean finish in any one application of the process. Some specific conditions follow:

Time Voltage Amperage Amp/em. (see) Volts 20-125 Amp./cm. 0.5-3.5 Seconds 20-400 Typical surface finishes obtained from prepolished surfaces-as rough as 30 microinches A.A. ranged from one to four microinches, A.A. (Arithmetic Average) with the mean being two.

-' Example I A pure titanium foil sample is made the anode in a vessel containing an electrolyte solution comprising 97% methyl alcohol, 2% sulfuric acid, and 1% hydrofluoric acid by volume. Percentages are calculated on 100% concentration of reagents.

The vessel is situated in an enclosure which is kept at -45 C. and the solution in the vessel is agitated slightly with a mechanical stirrer. The electrolyte circuit is completed by a platinum cathode. volts is applied across the circuit for a period of 80 seconds. This process resulted in a product which was scrupulously clean, and very brightly polished.

Example H To the same solution maintained at the same temperature as in Example I a titanium aluminum, vanadium alloy disc sample is made the anode in the electrolyte vessel. A voltage of 30 to 35 volts is applied between the anode disc and two platinum cathode plates, one on each side approximately one inch from the disc. Under these conditions 20 to 22 milliamps passed through the solution. The product which emerged had a polished surface which the electron microscope showed to be clean on a sub-micron scale.

What is claimed is:

1. A process for polishing and cleaning articles of titanium or titanium alloys comprising making the article the anode of a solution consisting of between 1 /z% and volume hydrofluoric acid, the remainder being methyl alcohol, and applying a voltage across said anode and a cathode while maintaining the solution at a temperature between 0 C. to 50 C. until such time that the cleaning and polishing treatment is complete.

2. The process of claim 1 wherein the voltage varies between 20 to 125 volts to pass between 0.5 to 3.5 amp./ cm. i

3. The process of claim 2 wherein the period of time for the ranges set forth therein is between 20 and 400 seconds.

4. The process of claim 1 wherein the electrolysis is carried out with stirring of the electrolytic solution.

References Cited UNITED STATES PATENTS U.S. C1. X.R. 

